Isoimmunization of female rabbits with a "soluble" sperm extract (100,000 g supernatant) results in sterility of females. This experiment has been repeated and confirmed during 1979-1980. Attempts to purify the "soluble" fraction antigen(s) by DEAE chromatography failed. Rabbits isoimmunized with the DEAE fractions (not bound or bound to DEAE) both were relatively fertile. This led to the discovery that the 0.015M PO4 ions used in the DEAE column precipitated significant material from the soluble fraction. The precipitate, on EM examination proved to be composed of very small membrane bound vesicles (approx. 250nm in diameter). During the coming year the following will be investigated: 1) The antifertility vaccine action of the 250nm vesicles from the "soluble" sperm fraction and the vesicle free soluble fraction. If the latter fails to have vaccine action, attention will concentrate on the vesicles, including attempts to solubilize them without destruction of antifertility vaccine action. 2) Immune sera from the infertile rabbits are presumed to contain antibodies against "antifertility" antigens of sperm. Such sera are being analyzed qualitatively with respect to number of sperm antibodies, their effect on sperm (e.g., agglutination, immobilization) and for localization (immunoelectron microscopy) and isolation (affinity chromatography) of sperm antigens.